Many natural resource industry activities involve determining the unknown or generally uncertain condition of a metallic casing or tubing string cemented or hung in a well bore. Casing and surface piping strings are used to handle fluids produced from or injected into an underground formation. Certain fluids, such as geothermal fluids, may present especially difficult corrosion and other fluid handling problems. Therefore, multi-layered construction (e.g., interior lined pipe having a barrier layer composed of a material such as concrete) is used to handle difficult, corrosive, erosive and scaling fluids.
The properties and condition of the underground casing and cement must be determined periodically to safely operate a well in a difficult application. Casing corrosion, cement failure, and casing shape distortions are some of the conditions that could cause unsafe operation. In-situ inspection is required for a cemented casing. Even when casing is not cemented, in-situ inspection is preferable.
In-situ cement and casing inspections are currently accomplished using various logging tools and instruments. Some of these tools project mechanical calipers to measure the inner or fluid contacting surface of the casing. Other instruments project pressure (e.g., ultrasonic) or electromagnetic wave spherically/radially outward and detect returning waves (i.e., waves affected by the casing/cement materials and interfacial conditions).
Existing well logging instruments are generally wire line supported. The wire line supports the hung instrument, supplies electrical power to actuate/generate the wave, and receives/transmits detected wave signals to an aboveground location. For example, hung electromagnetic instruments contain either 1) electrode potential/current sources, or 2) magnetic/induction field sources which produce electromagnetic waves which can interact with the metallic casing material to produce an induction signal. Similarly, hung sonic (including ultrasonic) type instruments generate a discrete pressure wave which Can interact with the casing/cement to produce an affected, but distinct reflected wave.
The instruments detect the induced or reflected signals/waves (i.e., perturbed signal) resulting from an interaction between the generated signal and the surrounding materials. The perturbed signal can represent one or more casing or cement conditions. Some wire line instruments have two or more receivers placed at different locations or have different orientations for transmitters and receivers so as to obtain radial and/or directional information on conditions at a given depth. Material types and interfaces are detected by "known" changes/perturbations in the perturbed (reflected or induced) signals. Signal perturbations may be to the wave speed, shape, frequency, direction, phase, and/or amplitude.
Normally, the initial (i.e., first radially outward) material/interfacial surface most strongly perturbs the signal, especially for sonic signals. Signals coming from subsequent interfaces (i.e., surfaces) or materials must be corrected for the perturbations caused by the initial material & interface (i.e., subsequent return signals must pass through intermediary signal perturbing materials/surfaces after first passing through them to reach the surface of interest). Thus, unless the initial materials and surfaces do not affect the wave (i.e., are relatively transparent), the condition of a casing must be determined from signal analyses which correct for known signal perturbing intermediary material/interfaces.
Besides the effects produced by the presence of intermediary materials of interest, existing instruments detect other signal perturbing influences. These influences include those identified with fluids present within the casing, fluids occupying the annular spaces between casing strings and between a casing string and the bore hole, and the underground formation. Other formation and casing conditions can also perturb signals. These fluid and other perturbing influences must also be considered in the analysis of the detected signal to accurately determine the condition of the casing.
Existing detected signal analysis methods generally use an idealized model with assumptions. The model treats the detected signals as perturbed by nominal or ideal signal perturbing conditions. For example, an idealized data analysis can assume a perfectly cylindrical casing, a non-interacting well bore fluid, and an infinitely thick formation layer (i.e., data are not affected by the formation boundaries). These models then provide idealized corrections for "known" perturbing factors.
However, the discrete casing or cement material/surface affected components of a signal (i.e., indicators) may be hidden in the data (e.g., signal characteristics obscured by a low signal to noise ratio) in difficult applications. In addition, some perturbing influences or factors may not be "known," making proper corrections difficult or impossible. For example, a deposit of a magnetically active scale having unknown magnetic properties perturbs induced electromagnetic signals carrying information with respect to the thickness of a metallic casing wall. Failure to accurately identify or "know" of the presence of an intervening signal perturbing material(s) compromises all other data. This has led to the inadequate detection of unsafe in-situ casing and cement conditions.
All of the current in-situ casing and cement condition analysis methods known to the inventors either will not work when an intervening material is present or they require a distinct intervening material signal to correct the perturbed signal, or they require a signal which is not affected by intervening materials, or they require an independent knowledge of factors which perturb the signal. The invention provides a method to obtain casing and cement condition information without knowledge of some of the signal perturbing factors, as well as providing other advantages as will become clear in light of the following description.